Messages in this thread | | | Date | Wed, 2 Nov 2022 07:44:57 -0400 | From | Steven Rostedt <> | Subject | Re: BUG: ib_mad ftrace event unsupported migration |
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On Wed, 2 Nov 2022 10:49:26 +0000 Leonid Ravich <leonid.ravich@toganetworks.com> wrote:
> after investigation looks like there is a broken assumption of rb_end_commit and rb_start_commit are on same CPU (no migration) during trace > looking in ib_mad trace in include/trace/events/ib_mad.h there is a call to create_mad_addr_info during TP assign which sleeps (at least under mlx5 ib driver)
Wait, WHAT?
> > so the scenario looks : > > rb_start_commit - buffer 0xffffa0984777e400 CPU 1 pid 1368 > switch from 1368 to 1605 > thread 1368 () is migrating from 1 to 0 > rb_end_commit - buffer 0xffffa09847c14a00 CPU 0 pid 1368
That's a MAJOR bug with the use case of tracepoints!
> > before starting throwing some patch into the the air I would like to align with you the approach we should take here. > > my suggestion here : > - ftrace infra should verify no migration happen (end and start happens on same CPU) in case not we will throw warning for the issue .
The scheduler should have. On entering the ring buffer code ring_buffer_lock_reserver() it disables preemption and does not re-enable it until ring_buffer_unlock_commit().
The only way to migrate is if you re-enable preemption. WHICH IS A BUG!
> - ftrace users will be responsible to avoid migration during TP assign (RDMA in my case)
Yes, it is responsible to do that. Just like it's up to users of spin_locks to not enable preemption and migrate while holding one. Entering the ring buffer code is the same as holding a spin_lock except that the ring buffer code allows for reentrancy.
> > please let me know what do you think
I think that whatever enabled preemption in the ib_mad trace event needs to be fixed.
-- Steve
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